4.1 Water systems

Sunlight supplies energy for evaporation; condensation releases latent heat; gravity drives drainage, runoff and flow downhill.

Separate phase change from movement. Solar energy changes liquid to vapour, while gravity moves precipitation through soil, rivers and groundwater toward lower elevation.

Sun warms a lake until water evaporates; after rain, gravity pulls runoff into a stream.

After precipitation, gravity drives drainage, run-off, streamflow and groundwater flow toward lower elevation; solar energy supplied the earlier phase-change energy.

Do not assign one driver to the whole cycle—different arrows have different causes.

Draw stores as boxes and flows as labelled arrows from source to destination, then state the system boundary.

At global scale, water matter is approximately closed: it changes state and location. Energy crosses the boundary as solar input and heat loss, so the system is open to energy.

A diagram can show ocean → evaporation → atmosphere → precipitation → river → ocean, with each arrow named.

The boundary determines which inputs and outputs count; a catchment is open even when the global cycle is nearly closed.

A store is an amount, not a process; flows need direction and often a rate.

Oceans hold about 96.5% of Earth’s water; ice and groundwater are the next large stores, while rivers, lakes, air and organisms are tiny fractions.

Use the order of magnitude to interpret access: global abundance does not mean freshwater is easy to reach, clean or renew quickly.

A lake may be vital locally but still contain only a minute share of global water compared with the ocean.

Most water is saline or locked in slow stores; accessible freshwater is a small, uneven fraction.

A percentage of global water is not a measure of local availability or sustainable supply.

Transformations change water state; transfers move water, and infiltration is entry into soil while percolation is movement through it.

Evaporation, transpiration, condensation, freezing, melting and sublimation involve a change of state or release of water vapour. Advection moves vapour, liquid droplets or ice crystals horizontally; precipitation, surface run-off, streamflow and groundwater flow transfer water between locations.

Rain infiltrates the surface, then percolates through porous soil toward groundwater.

Advection is wind-driven horizontal transfer of water in the atmosphere; condensation changes vapour into liquid and precipitation transfers water from atmosphere to surface.

Infiltration and percolation are sequential but not synonyms.

Land use changes interception, evapotranspiration, infiltration and drainage, which can alter peak flow, recharge and flood risk.

Deforestation can reduce canopy storage; compaction and urban surfaces reduce infiltration and speed runoff. Irrigation may raise evapotranspiration or runoff depending on soil, rate and drainage.

Replacing permeable ground with roads shortens the time to peak flow and can increase flash flooding after the same rainfall.

Impermeable surfaces reduce infiltration and groundwater recharge while faster surface run-off shortens lag time and can raise peak discharge and flash-flood risk.

One land-use change can affect several flows; do not assume every deforestation site has the same direction or size of effect.

For a defined water body and time period, storage changes according to its inputs, natural outputs and harvesting.

ΔS is change in storage, I is total input, O is natural output and H is harvested water. All quantities must use the same volume-per-time unit and the same system boundary.

Example: a lake receives 180 million m³ yr⁻¹ and loses 150 million m³ yr⁻¹ naturally. At steady state, H = 180 − 150 = 30 million m³ yr⁻¹. This is an arithmetic maximum; a sustainable quota may be lower to protect ecological flows and allow for drought and uncertain recharge.

A balanced annual budget does not guarantee sustainability in every season: state the boundary, time period, units and ecological allowance.